A pattern rule contains the character `%' (exactly one of them) in the target; otherwise, it looks exactly like an ordinary rule. The target is a pattern for matching file names; the `%' matches any nonempty substring, while other characters match only themselves.
For example, `%.c' as a pattern matches any file name that ends in `.c'. `s.%.c' as a pattern matches any file name that starts with `s.', ends in `.c' and is at least five characters long. (There must be at least one character to match the `%'.) The substring that the `%' matches is called the stem.
`%' in a dependency of a pattern rule stands for the same stem that was matched by the `%' in the target. In order for the pattern rule to apply, its target pattern must match the file name under consideration, and its dependency patterns must name files that exist or can be made. These files become dependencies of the target.
Thus, a rule of the form
%.o : %.c ; command...
specifies how to make a file `n.o', with another file `n.c' as its dependency, provided that `n.c' exists or can be made.
There may also be dependencies that do not use `%'; such a dependency attaches to every file made by this pattern rule. These unvarying dependencies are useful occasionally.
A pattern rule need not have any dependencies that contain `%', or in fact any dependencies at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. See section Defining Last-Resort Default Rules.
Pattern rules may have more than one target. Unlike normal rules, this
does not act as many different rules with the same dependencies and
commands. If a pattern rule has multiple targets, make knows that
the rule's commands are responsible for making all of the targets. The
commands are executed only once to make all the targets. When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
make worries only about giving commands and dependencies to the file
presently in question. However, when this file's commands are run, the
other targets are marked as having been updated themselves.
The order in which pattern rules appear in the makefile is important since this is the order in which they are considered. Of equally applicable rules, only the first one found is used. The rules you write take precedence over those that are built in. Note however, that a rule whose dependencies actually exist or are mentioned always takes priority over a rule with dependencies that must be made by chaining other implicit rules.